The present invention relates generally to an information recording medium and its manufacturing method. More specifically, the invention relates to a phase change recording medium, which is an optical recording medium having a phase change type optical recording layer irradiated with light beams for recording and/or reproducing information and which does not need any initial crystallizing steps for crystallizing the recording layer after the recording layer is deposited.
In a phase change type optical recording medium irradiated with optical beams for recording and/or reproducing information, there are advantages in that the medium has a large capacity, high-speed accessibility and medium portability, and that it is possible to more inexpensively provide a CD interchangeable drive than competitive magneto-optical media since its reproduction principle is reflectance change type which is the same as those of CDs. In addition, there are advantages in that it is possible to easily increase the density of the medium since the medium has an excellent signal quality, and that the medium has a high recorded-data transfer rate.
The phase change recording medium is able to record information by forming a recording mark and to erase information by erasing the recording mark. The recording mark is formed by allowing a recording layer to be irradiated with a light beam of a recording level to be molten to be in a random state, and then, by cooling the recording layer in a shorter time than a recording-layer crystallizing time to quench the random state to room temperature to form an amorphous recording mark. On the other hand, the recording mark is erased by irradiating the recording layer with a light beam of an erase level to raise the temperature of the recording layer to a temperature of less than its melting point and not less than its crystallizing temperature in a longer temperature raising time than the recording-layer crystallizing time to crystallize the recording layer. In addition, the reproduction of information is carried out by utilizing the difference in reflectance between a crystal and an amorphous material.
Since the phase change recording medium can record whether its state is amorphous or crystalline before a recording operation, there is an advantage in that an overwrite operation can be carried out by one beam.
As an example of the phase change recording medium, there is an optical disc. A typical optical disc has a structure (a four-layer structure) wherein a lower dielectric layer, a recording layer, an upper dielectric layer and a reflective layer are sequentially laminated on a polycarbonate substrate, the header part of which is pre-formatted and the data part of which is pre-grooved. Moreover, a counter substrate is applied to the reflective layer via an adhesive layer, or a label is applied to the reflective layer.
As the recording layer, there is used a thin film of a chalcogen metal compound, e.g., GeSbTe, AgInSbTe or InSbTe, which suitably includes a very small amount of Cr, V, N or the like.
The dielectric layer and the reflective layer serve to prevent the oxidation of the recording layer, to prevent the deterioration of the recording layer due to accumulated overwrite, to adjust the thermal response of the recording layer during a recording operation, and to optically enhance the recording layer during a reproducing operation. In particular, with respect to the optical enhancement effect, the lower dielectric layer can increase the variation in reflectance by the multiple interference effect between the substrate and the recording layer, and the upper dielectric layer can increase the variation in reflectance by the multiple interference effect between the recording layer and the reflective layer, so that the optical enhancement effect can improve signal quality.
The phase change recording medium described above is applied to various information storage system, such as CD-RW (compact disc-rewritable) and DVD-RAM (digital versatile disc-random access memory). In future, it is expected to increase the storage capacity of the phase change recording medium, to accelerate the transfer rate thereof, and to lower the price thereof.
However, the inventor has recognized that there are various problems to be solved in phase change recording media, after having continued to make a unique study thereof. These problems will be enumerated below.
(Problem on Technique for Improving Transfer Rate)
First, the problem of transfer rates in the prior art will be described. The request for the acceleration of data transfer rates is high similar to other recording media. However, in the phase change recording media, it is required to shorten the time to crystallize a recording layer in order to improve the data transfer rate during a recording operation. Because the acceleration of the data transfer rate means the shortening of the time for an optical spot to pass. In order to shorten the crystallizing time, it has been proposed to add a very small amount of an element other than principal elements constituting the recording layer to the recording layer, and/or to provide a crystallization controlling seed layer underlying the recording layer. However, this is not sufficient for the shortening of the crystallizing time, so that the data transfer rate of the phase change recording medium is limited to tens Mbp (mega bit per second) or less.
(Problem on Reduction of Producing Costs)
Typically, a conventional method for producing a phase change recording medium comprises:    (1) Master Disc Mastering Process;    (2) Stamper Producing Process;    (3) Substrate Forming Process by Injection;    (4) Film Attaching Process by Sputtering;    (5) (Bonding process of a counter substrate if necessary);    (6) Initial Crystallizing Process; and    (7) Verifying Process.
Among a series of these processes, “(6) Initial Crystallizing Process” is a process for crystallizing an as-deposited phase change recording layer (in a state as deposited) on the whole surface of a disc. The reason why this process is provided is that the as-deposited amorphous recording layer takes a very long time required to recording unlike an amorphous mark formed by an optical recording operation. Therefore, the conventional phase change recording medium is not used as-deposited, so that it is required to crystallize the recording layer at the initial crystallizing step.
For example, in the case of an optical disc, at the “initial crystallizing step”, there is adopted a system for rotating a disc at a relatively low speed while irradiating the disc with elliptical laser beams extending in radial directions of the disc at a high power to feed the beams in radial directions at a shorter pitch than the major axes of the elliptical beams to gradually anneal the recording layer to crystallize the recording layer. Although the time required for the crystallization depends on the diameter of the disc, the linear velocity during initialization, and the feed pitch, it takes at least several minutes including the focusing time, so that the productivity is very bad. Since an actual producing line is designed so that a tact per disc is several seconds, tens initializing systems must be arranged. Therefore, there are problems in that the costs for the systems are high, that it is required to ensure the area for installing the systems, that it is required to carry out the maintenance of the systems, that the productivity of the recording medium is low, and that the producing costs increase.
(Problem on Degree of Freedom for Selection of Structure of Recording Medium)
Another problem of conventional phase change recording media is that the degree of freedom for the selection of the structure of the medium is limited. That is, although most of conventional phase change recording media are set so that Rc (the light reflectance of a crystal part) is higher than Ra (the light reflectance of an amorphous recording mark), this results from the fact that it is required to carry out the initial crystallizing step as described above.
That is, when the initial state of the medium is crystalline, the Rc is set to be higher than the Ra, so that the reflectance before recording is high, the reflectance of address parts and data parts in the initial state is high, the qualities of header signals and servo signals are improved, and the stability of servo is good.
However, if the limitation that the initial state of the medium is crystalline is removed, the reflectance of the amorphous mark (Ra) can be freely designed so as to be higher or lower than the reflectance of the crystal part by selecting the thickness and material of each layer.
However, since the conventional phase change recording medium has a high Rc, there are disadvantages in that the absorptivity (Ac) of the crystalline state can not be so high, so that the recording sensitivity is bad, that it is difficult to adjust the absorptivity required to record the mark length, and that it is required to carry out the initializing process, so that the costs for the producing process are high. The “adjustment of absorptivity” means that the absorptivity Ac of the crystalline state is set to be higher than the absorptivity (Aa) of the amorphous state in order to allow the film temperature of the crystal part to be equal to that of the amorphous part during fusion in view of the latent heat of fusion, i.e., in order to reduce the overwrite jitter.
In a medium having a so-called high to low structure (which will be hereinafter briefly referred to as a “HtoL structure”) wherein Rc>Ra, there is no optical layer other than at least the recording layer, and Ac<Aa is automatically established in the total reflection type film structure, so that it is not possible to adjust the absorptivity. As methods for adjusting the absorptivity in the HtoL structure wherein Rc>Ra, there are methods for causing a reflective film to be semitransparent (thin), for providing a light absorbing layer between the recording layer and the reflective layer, and so forth. However, there is a problem in that the ration Ac/Aa of the absorptivity is about 1.2 at the most even by these methods, so that these methods are not suitable for high linear velocity operations wherein it is required to adjust the absorptivity.
On the other hand, the LtoH (Low to High) medium, wherein the Rc is adjusted to be lower than the Ra, has the merits of having a high recording sensitivity and being easy to adjust the absorptivity, so that the LtoH medium is expected to be the main current of optical discs in the next generation. In particular, a medium having a five-layer film structure, wherein a thin semitransparent film of a metal is arranged between the above described substrate of the four-layer film structure and a lower dielectric layer, can be designed so as to have an Ac/Aa of 1.5 or more by suitably selecting the thickness of upper and lower interference films, and the crystal part thereof has a high recording sensitivity, so that such a medium is suitable for high linear velocity operations.
However, even in such an LtoH medium, the Rc decreases as the Ac/Aa is set to be higher and as the reproducing CNR is set to be higher, so that there are problems in that it is difficult to read address parts if the initial crystallizing step is carried out similar to the medium having Rc>Ra, and that it is difficult to read the servo signals of data parts in a state before recording.
(Problem on Increase of Storage Capacity)
As techniques for improving the recording density of a phase change medium, there are techniques for decreasing the wavelength of a light source, for increasing the NA of an objective lens, for applying a super resolution thin-film and so forth. On the other hand, as means for improving the storage capacity of the medium without the need of the improvement of the recording density of the medium, a single-sided double-layer disc is provided. The single-sided double-layer disc is designed to record and/or reproduce data by only adjusting the focal position of light beams on a double-layer recording layer apart from the plane of incidence for the same light beam by about tens μm. Since it is not required to turn a disc over, it is considered by the user that the single-sided double-layer disc has substantially the same performance as that of a single-sided single-layer disc having a recording density substantially twice as large as that of the single-sided double-layer disc. As a reproduction only DVD, there is known a single-sided double-layer disc which is known as a common name DVD-9. However, it has been considered that since the transmittance of a rewritable DVD is insufficient by one recording layer, light beams do not sufficiently reach a recording layer arranged at the bottom with respect to the incident side of light beams, so that it is difficult to record and/or reproduce data.
However, in ISOM (International Symposium on Optical Memory) '98, Technical Digest, pp. 144-145 (Th—N-05), it has been suggested that it is possible to form a single-sided double-layer even in the case of a rewritable phase change medium. The points of this technique are that the transmittance of a first recording layer part is increased to about 50% so that light beams sufficiently reach a second recording layer part arranged at the bottom when the first recording layer part and the second recording layer part are arranged in that order from the incident side of light beams, that the reflectance of the second recording layer part is set to be higher, i.e., the transmittance thereof is lower, in order to maintain the balance of servo signals and regenerative signals from the first and second recording layer parts, and that the absorptivity Ac of the crystal part is set to be higher than the absorptivity Aa of the amorphous part in both the first and second recording layer parts in order to reduce overwrite jitters.
In order to satisfy the above described setting, the first recording layer part has a three-layer construction which has a so-called High to Low structure (which will be hereinafter briefly referred to as a “HtoL structure”), wherein the reflectance Rc of the crystal part is higher than the reflectance Ra of the amorphous part, and which has no reflective film, and the second recording layer part has a five-layer construction which has the LtoH structure, wherein the reflectance Rc of the crystal part is lower than the reflectance Ra of the amorphous part, a thin Au semitransparent film underlying the LtoH structure, and a thin Al—Cr reflective film on the top of the LtoH structure.
In this construction, with respect to the reflectance of each recording layer part viewed from the incident side of light beams, the reflectance of the first recording layer part is 9% of that of the crystal part and 2% of that of the amorphous part, and the reflectance of the second recording layer part is about 3% of that of the crystal part and about 9% of that of the amorphous part. Therefore, if the single-sided double-layer phase change medium is initial-crystallized in accordance with the conventional producing process, the initial reflectance of the address part and data part is about 9% in the first recording layer and about 3% in the second recording layer. This initial reflectance is far lower than, e.g., 15% to 25% of the single-sided single-layer DVD-RAM standard. At the initial reflectance of the first recording layer, it is possible to reproduce address signals and servo signals of the data part somehow if the reproducing power is increased. However, the reflectance of the second recording part is too low, so that it is difficult to reproduce both of address signals and servo signals.
In addition, the common problem of single-sided double-layer media, which are not limited to the above described rewritable media, is that the initial crystallizing step is complicated. That is, if each of the first and second recording layer parts is initial-crystallized, it is required to carry out double steps to deteriorate the productivity and producing costs.